Cucumber mosaic virus (CMV) is a single-stranded (+) RNA plant virus which has a functionally divided genome. The virus genome contains four RNA species designated RNAs 1-4; and for CMV strain Q 3389 nucleotides (nt), 3035 nt, 2193 nt and 1027 nt, respectively (Peden and Symons, Virology, 53, 487-492, 1973; Gould and Symons, 1982 Eur. J. Biochem., 126, 217-226; Rezaian et al., 1984 Eur. J. Biochem. 143, 227-284; Rezaian et al., 1985 Eur. J. Biochem. 150, 331-339). Only RNAs 1-3 are required for infectivity (Peden and Symons, 1973) because the coat protein, which is encoded by RNA 4, is also encoded by RNA 3. Translations of CMV RNAs yield a 95KDal polypeptide from RNA 1, a 94kDal polypeptide from RNA 2, (Gordon et al., 1983 Virology 123, pp 284-295) and two polypeptides from RNA 3: its 5' end encodes a 35KDal polypeptide, and its 3' end encodes a 24.5kDal polypeptide (Gould and Symons, 1982). The 24.5kDal polypeptide is identical to that encoded by RNA 4 and is the coat protein.
The CMV coat protein gene does not contain the signals necessary for its expression once transferred and integrated into a plant genome. It must be engineered to contain a constitutive promoter 5' to its translation initiation codon (ATG) and a poly(A) addition signal (AATAAA) 3' to its translation termination codon. Several promoters which function in plants are available, but we believe that the best promoters are the constitutive promoters from CaMV, the Ti genes nopaline synthase (Bevan et al., 1983 Nucleic Acids Res. II 369-385) and octopine synthase (Depicker et al., 1982 J. Mol. Appl. Genet. 1, 561-564), and the bean storage protein gene phaseolin. The poly (A) addition signals from these genes are suitable for our purposes as well.